One class of well-known dampers and shock-absorbers uses a volume of hydraulic fluid as the working medium to create damping forces to control or minimize shock and/or vibration. Typically, the damping forces are generated by pressures resisting movement between operative components of the damper or shock absorber.
Another class of devices employed to minimize shock and/or vibration comprises devices that include a field controllable material such as a magnetorheological (MR) medium which may comprise MR fluid or MR powder. Such devices referred to as “MR devices” may be of the “rotary-acting” or “linear-acting” variety. Known MR devices include linear dampers, rotary brakes, and rotary clutches for example. Each MR device employs an MR medium comprised generally of soft-magnetic particles dispersed within a carrier. Typical particles include carbonyl iron, and the like, having various shapes, but which are preferably spherical and have mean diameters of between about 0.1 μm to about 500 μm. The carrier is most frequently a fluid among the group of fluids including low viscosity hydraulic oils, and the like. In operation, these MR fluids exhibit a thickening behavior (a rheology change) upon being exposed to a magnetic field. The higher the magnetic field strength in the fluid, the higher the damping/restraining force or torque that can be achieved within the MR device. The magnetic field is generated by supplying a current to a coil that is located proximate a pole piece.
FIG. 1 illustrates an exemplary MR damper of the type disclosed in U.S. Pat. Nos. 6,151,930 and 5,284,330 commonly assigned to the assignee of the present invention, Lord Corporation of Erie, Pa. the disclosures of which are incorporated herein by specific reference. Damper 10 may be used in a variety of applications. For example, a plurality of dampers may be used to support an engine on a vehicle frame or to suspend a drum in a washing machine cabinet or housing. The damper 10 of FIG. 1 includes cylindrical housing 12 that defines housing chamber 14 and piston member 16 disposed in the chamber and adapted to be translated linearly along axis 17 through the housing chamber. An attachment stem 28 is made integral with one end of the housing 12 in a conventional manner and the free end of the stem is in turn fixed to a frame such as the frame of a machine or engine by a conventional connection member such as a bolt for example.
The piston body 18 and the housing wall 20 are made of a magnetically permeable material such as a soft magnetic steel for example and the piston body and housing comprise pole pieces that define the path of magnetic field 22 represented in dashed font in FIG. 1. The piston body includes a piston rod 32 that is securedly fixed to the piston, and the free end of the rod is in turn fixed in a conventional manner proximate the member, device or system that is the primary source of vibratory displacement such as an engine or an enclosure for a washing machine drum for example.
In the exemplary MR damper 10 of FIG. 1, a volume of a field controllable medium such as a magnetorheological medium is contained in an absorbent matrix 30 which is wrapped around the piston body 18. The absorbent matrix may include polyurethane foam for example. In an alternate embodiment, illustrated in incorporated by reference U.S. Pat. No. 5,284,330 the field controllable material is not retained in an absorbent matrix. A substantial portion of the housing chamber is filled with a volume of the field controllable material and during operation the field controllable material is displaced from one end of chamber 14 to the opposite end of chamber 14 as it is entrained in a gap between the piston body and housing wall during axial displacement of the piston through the housing chamber.
A magnetic field generating means 24 in the form of a coil is mounted on the piston body 18 to be movable with the piston as it is reciprocatingly displaced axially through the housing 14. The field generating means alters the rheology of the field responsive medium in proportion to the strength of the field. Wires 26 connect the coil comprising the field generating means to a controller, not shown in FIG. 1. The controller is disclosed schematically in FIGS. 3a-3e and the controller will be described in greater detail hereinafter.
During operation of damper 10 the field controllable medium becomes increasingly viscous with increasing field strength and provides a shear force to resist relative movement between housing and piston members 12 and 16. When the pole pieces are energized by magnetic field 22 the controllable fluid changes rheology in the matrix 30 located between the movable member 18 and the housing 12. In use, the circumferentially extending coil 24 generates a magnetic field 22 that acts on the pole pieces 18 and 20 and the field controllable medium contained by the matrix 30. The coil generates a magnetic field in response to the current supplied to the coil 24 by the controller. The resistive force produced by changing the field controllable medium's rheology can be varied by changing the magnetic field strength which in turn is controlled by the amount of current supplied to the field generating means 24 by the controller.
When the damper is used to suspend washing machine drums from the washing machine cabinet, the magnitude of damping supplied is adjusted for the different washing cycles. Turning now to FIG. 2 which is a graph representing the transmitted washing machine forces during the range of washing machine drum spin speeds, the supplied damping is increased between points A and B on FIG. 2 which defines a the range of speeds that occur during machine resonance. For example, the washing machine represented by exemplary FIG. 2 experiences resonance as the drum passes through the speed range between minimum and maximum spin speed limit points A and B which may be 100 and 200 rpm for example. Resonance occurs during periods of machine drum acceleration and deceleration when the drum is spinning in the speed range between threshold points A and B. The supplied damping is reduced outside the spin speed range between points A and B.
The adjustable damping provided to washing machines by damper 10 is a considerable improvement over the single constant damping supplied by passive damping devices. If the damping was not supplied through resonance as the washing machine rotated through the speeds that produce a resonance condition, the produced vibratory forces could cause the washing machine to “walk” from its operating location. Also, the vibratory forces could damage the machine.
During a loss of power to the washing machine or other apparatus supported by one or more dampers 10, the required current is not supplied to the field generating means 24 to control damper 10. As a result, the requisite damping forces can not be supplied by damper 10 to control the vibration produced during resonance experienced as the washing machine rotates through the spin speeds between points A and B. Thus the washing machine passes through resonance undamped which could cause the washing machine to vibrate from its desired operating position and the machine could be damaged.
The foregoing illustrates limitations known to exist in present damping devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative system and method whereby vibration is controlled in the event there is a loss of power to an apparatus. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.